Optical analysis methods such as interferometric methods deliver light onto a sample of interest, and further require collection of a portion of the light returned from the sample. Due to the size and complexity of many light sources and light analysis devices, they are typically located remotely from the sample of interest. This is especially apparent when the sample of interest is an internal part of a larger object, such as biological tissue inside of a living organism. One method of optically analyzing internal parts is to guide light from a remote light source onto the sample using a thin optical fiber that is minimally disruptive to the normal function of the sample due to the diminutive cross-section of the optical fiber. An example of such a method is the optical analysis of a luminal organ, such as a blood vessel, using a fiber-optic catheter that is connected on one end to a light source outside of the body while the other end is inserted into the vessel.
A significant barrier to conducting optical analysis of internal regions, such as lumens, is the design and low-cost manufacture of miniature optical devices for focusing or collimating light. Many types of optical analysis, such as imaging and spectroscopy, require that the light incident on the sample be focused at a particular distance or substantially collimated. Since light radiating from the tip of a standard optical fiber will diverge rapidly, a miniature optical system can be coupled to the fiber to provide a focusing or collimating function. Additionally, it is often desirable to analyze a sample location that is not directly in line with the optical axis of the fiber, such as the analysis of the luminal wall of a thin blood vessel. In these situations, a means for substantially altering the direction of the light is used in addition to a means for focusing or collimating the light radiating from the tip of an optical fiber.
Many methods have been previously described for manufacturing miniature optical systems suitable for attachment to an optical fiber that provide some of the functionality described above. These methods generally provide a beam focusing means using one of three methods: 1) using a graded-index (GRIN) fiber segment; 2) directly shaping the fiber tip into a lens; or 3) using a miniature bulk lens. A beam directing means is generally provided using one of four methods: 1) using total internal reflection (TIR) of light from the angled end face of the fiber using an angled, reflective surface; 3) using a miniature bulk mirror; or 4) using a reflective coating on the fiber tip. These methods, however, have numerous inherent limitations, including excessive manufacturing cost, excessive size, or insufficient freedom to select the focal spot size and focal distance.
There are many miniature optical systems known in the art that can be used for analysis of internal luminal structures. Each optical system can be conceptually divided into a beam focusing means and a beam directing means. Light is passed from an external light source to the internal lumen through one or more optical illumination fibers, which may be single mode or multimode in nature. The illumination fiber is in communication with the miniature optical system, which focuses and directs the beam into the luminal wall. Light is returned from the lumen to an analysis apparatus outside the body using the same fiber, or using other fibers co-located with the illumination fiber. In one type of miniature optical system design, the focusing means and directing means are performed by separate optical elements. In another type of design, the focusing means and directing means are performed by the same element.
Several features of existing optical systems are undesirable. For example, in some devices all of the optical elements must be of a diameter similar to the optical fiber (the diameter often being similar to 125 μm) in order to minimize the overall system size. This greatly reduces the options available for selecting the focusing element, beam expander, and beam director and therefore limits the range of focal spot sizes and working distances achievable by the design. Additionally, these extremely small elements are fragile, difficult to handle, and prone to break during manufacturing and operation. Third, in many embodiments an air gap must be provided in order to use TIR for beam redirection. This requires a tight seal to be maintained between the fiber and the other element to maintain the air gap. This can be problematic when the device is immersed in water, blood, or stomach acid, or when the device is rotated or translated at high speed in order to form an image. Fourth, GRIN focusing elements have refractive index profiles that are rotationally symmetric, making it impossible to correct for cylindrical aberrations induced on the beam. The overall effect of these drawbacks is that certain miniature optical systems are expensive, difficult to manufacture, prone to damage, and do not produce a circular output at the focal plane.
In addition to the drawbacks listed above, conventional lensed surfaces can only provide small radii of curvature and are largely limited to spherical geometries. Additionally, the beam cannot expand to a size significantly larger than the single mode fiber diameter (often 125 μm) at any point in the optical system. These limitations result in a lens system with a limited working distance and significant spherical aberrations.
As described above, there are significant limitations to currently known miniature optical systems used for conducting optical analysis or imaging. Accordingly, a need exists for optical elements that overcome the limitation of existing optical devices.